System and method for man overboard incident detection

ABSTRACT

A system and method for detecting a man overboard incident on structures such as cruise vessels and oil rigs. The system includes at least two opposed imaging devices which record video streams of a detection cuboid within an overlapping region of view volumes for the imaging devices. The imaging devices are located at the lowest deck of the structure and monitor a fall that passes through the cuboid. Identified objects within the video streams are paired, their conformance is determined, and real world information such as size, trajectory, and location is determined.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of prior U.S. Patent Application No. 62/057,639, filed Sep. 30, 2014, which is hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates generally to image processing and computer vision and more particularly to man overboard detection systems.

BACKGROUND OF THE INVENTION

“Man overboards”—the event of a person falling from some height into the water, are serious concerns. There are many elements which make man overboard events dangerous. They often occur under adverse conditions, such as at night or in high seas. If the air or water temperature is cold, a more than temporary exposure can lead to hypothermia and death. If the fall is from a great height, such as the top of a cruise vessel or oil platform, the fall can injure, knock unconscious, or even kill the fall victim. For reasons such as these, hobbyist, commercial, and military vessels all have protocols for rescuing people who have fallen into the water. However, given the conditions in which a man overboard event happens, protocols are no substitute for immediate detection and speed of recovery.

The incidence of man overboard events on ocean-going passenger cruise vessels has been of such increasing concern recently that it led to a Congressional mandate. In 2010, Congress passed the Cruise Vessel Safety and Security Act (“CVSSA”) to address the issue of properly detecting persons who fall overboard. The CVSSA requires that “the vessel shall integrate technology that can be used for capturing images of passengers or detecting passengers who have fallen overboard, to the extent that such technology is available.”

Such technology is virtually non-existent, and where extant, is crude at best. Man overboard detection systems are confronted with the formidable challenges of being automatic or semi-automatic, performing in and out of daylight, and operating with high levels of certainty and extremely low false alarm rates at all times and in all waters. Moreover, they must perform under adverse weather conditions and cope with at least moderate vessel movement such as pitching and listing. An improved system and method for detecting man overboard incidents is needed.

SUMMARY OF THE INVENTION

A system and method for detecting a man overboard incident on above-water structures such as cruise vessels and oil rigs. The system includes at least two opposed imaging devices which record video streams of a detection within an overlapping region of view volumes for the imaging devices. The imaging devices are located at the lowest deck of the structure and monitor a fall that passes through the cuboid. Identified objects within the video streams are paired, their conformance is determined, and real world information such as size, trajectory, and location is determined.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring to the drawings:

FIG. 1 is a side elevation view of a vessel showing camera coverage and detection cuboids of a system for man overboard incident detection on the vessel;

FIG. 2 is a top plan view of the vessel showing the camera coverage and detection cuboids;

FIG. 3 is a functional block diagram showing steps of the system;

FIGS. 4 and 5 are side elevation views of the vessel showing alternate camera coverage and detection cuboids;

FIG. 6 is a top plan view of the vessel showing the alternate camera coverage and detection cuboids of FIGS. 4 and 5;

FIGS. 7 and 8 are top plan views of a stern of the vessel showing alternate camera coverage embodiments; and

FIG. 9 is a top plan view of a bow of the vessel showing an alternate camera coverage.

DETAILED DESCRIPTION

Reference now is made to the drawings, in which the same reference characters are used throughout the different figures to designate the same elements. To address man overboard (“MOB”) incidents on cruise vessels, oil platforms, and other above-water structures, an MOB incident detection system 10 is disclosed herein and illustrated in the various drawings. FIG. 3 shows a functional block diagram illustrating steps of the system 10. As shown in FIGS. 1 and 2, the system 10 uses at least two opposed video cameras 11 and 12 mounted to a vessel 13. The cameras 11 and 12 are located and oriented so as to view and image the last portion of a fall. Any object falling from the vessel 13, irrespective of from which deck the falls begins, will necessarily pass a lowest deck 14. The cameras 11 and 12 are thus placed to capture and image outside of the vessel 13 from the lowest deck 14 down to a waterline 15. By concentrating the cameras 11 and 12 in such manner, the detection resolution of each camera 11 and 12 is spread over a volume that all falls must pass through. Hence, the number of pixels on a falling object (or “pixels on target” or “POT”) is maximized for each of such opposing camera pairs while also ensuring that a necessary and sufficient number of snapshots are captured of the fall. This allows the system 10 to make a reliable determination as to whether an MOB event has taken place or not. Further, by vertically limiting the detection volume to this extent, the system 10 can calculate monitoring time—or fall time—and measure speed at the ceiling of the detection volume and at the waterline, and determine from which deck the fall originated, which information is useful for the crew of the vessel 13 when conducting an investigation.

Briefly and generally, salient elements, features, and characteristics of a preferred embodiment of the MOB incident detection system 10 include, but are not limited to:

-   -   i) A continuous series of opposing video camera pairs located         and oriented to view and image the last portion of any and         falls;     -   ii) A method of exploiting such camera pair to transform image         space into world space, thereby leading to insights into the         real-world object sizes and location;     -   iii) A method of seeking conformance of objects in both camera         views through the height and width of bounding boxes for the         objects;     -   iv) A method of transforming image space into world space as in         (ii), above, to track and compare segments of a fall trajectory         to determine a man overboard event; and     -   v) Leveraging information gathered in (ii) and (iii) to qualify         the determination made in (iv).

The above items are not an exhaustive list of features of the system 10, but describe generally some of the features to better prepare the reader with the description herein. It is noted here that, throughout this description, the term “image space” is used to mean the space, or coordinate system for the space, imaged by a camera. Likewise, “world space” is the space, or coordinate system for the space, which is the real world. For example, an object in image space may have a height of ten pixels, while, that height may correspond to a height of six feet in world space. Indeed, a world coordinate system useful in (ii) and (iv) above is a three-dimensional Cartesian coordinate system attached to the side of the vessel 13 with the origin located between the opposing pair of cameras 11 and 12 at the level of the opposing pair of cameras 11 and 12.

The system 10 leverages multiple cues to reliably recognize an MOB event. Such cues include adherence to an expected world space fall path or trajectory, time-stamped speed and acceleration values for an object, and world space size determinations.

Referring now to FIGS. 1 and 2, the system 10 is described in the context of an exemplary vessel 10 equipped with the cameras 11 and 12. The example cruise vessel 10 has nine passenger decks from which overboard falls could initiate. The lowest deck, Deck 3, is 12 meters (39 feet) above the waterline 15 while the highest deck, Deck 11, is 33 meters (108 feet) above waterline 15. Deck 3 is the lowest deck from which a fall could initiate as there are no decks below Deck 3 which also have access to an exterior 16 of the vessel 10. Correspondingly, in this description, it is understood that the term “deck” is used to mean only those levels from which a fall could actually occur because such decks provide access to the exterior 16. FIGS. 1 and 2 show a starboard side of the vessel 10.

The cameras 11 and 12 are mounted to the exterior 16 of the vessel 10, either directly or on a pole or other mounting platform. The camera 11 is a forward camera because it is mounted forward of the camera 12 and is toward the bow 19 of the vessel 10. The camera 11 is oriented aft, toward the camera 12. The camera 12 is an aft camera because it is mounted rearward of the camera 11 and is toward the stern 20 of the vessel 10. The camera 12 is oriented fore, toward the camera 11. Each of the cameras 11 and 12 is preferably a VGA (640×480) resolution thermal video camera. Thermal cameras are advantageous within the system 10 because they are passive, a feature not met by visible-light cameras when operating at dark hours or by LIDAR sensors in an opposed arrangement without additional preparatory steps. The cameras 11 and 12 are both rotated ninety degrees, so that each has their horizontal field of view (“HFOV”) oriented vertically and their vertical field of view (“VFOV”) oriented horizontally, when the vessel 10 demonstrates zero pitch and roll. In most cameras, the HFOV is typically the larger field of view, and so the rotated orientation provides a larger field of view vertically than horizontally. This rotated orientation allows the system 10 to more frequently meet vertical spatial coverage requirements between Deck 3 and the waterline 15 without unduly limited horizontal spatial coverage requirements, as will be described. This rotated orientation also sets scan-line functionality in a vertical direction with respect to the vessel 10; the HFOV of each camera 11 and 12 is aligned with the scan line, and the VFOV of each camera 11 and 12 is perpendicular to the scan line.

Each camera 11 and 12 has a pyramidal view volume which images in the field of view. Camera 11 has a view volume 21 extending aft, and camera 12 has a view volume 22 extending fore. The view volumes 21 and 22 overlap, as shown in FIGS. 1 and 2. Within this overlap, a detection cuboid 23 is established. The cuboid 23 occupies a portion of the overlap, and defines an MOB detection volume for the pair of cameras 11 and 12. Different camera characteristics and arrangements will cause varying amounts of the overlap to be occupied by the cuboid.

The cuboid 23 is defined by several boundaries. Referring particularly to FIG. 1, the cuboid 23 has a ceiling 24 and an opposed base 24. The ceiling 24 is defined by, and thus aligned with and extends along, the upper boundary of each view volume 21 and 22. The cameras 11 and 12 are oriented toward each other but also downwardly at an angle, so that the view volumes 21 and 22 are generally angled downwardly. As such, the upper boundaries of each view volume 21 and 22 are coincident and define the ceiling 24 of the cuboid 23. Opposed from the ceiling 24 is the base 25, which is defined by the waterline 15. Thus, the cuboid 23 extends vertically along a height H_(cuboid) from the level of the cameras 11 and 12, at the lowest deck 14, to the waterline 15.

The cuboid 23 also has a length L_(cuboid) aligned between the bow 19 and the stern 20. The length L_(cuboid) of the cuboid 23 will vary as needed to satisfy the length criterion of the cuboid 23, as will be described herein. However, the length L_(cuboid) of the cuboid 23 extends from a vertical plane at one end defined by the intersection of the view volume 21 with the waterline 15 and at the other end defined by the intersection of the opposed view volume 22 with the waterline 15. The cuboid length L_(cuboid) shall not be so great that it forces either camera 11 or 12 to detect a target beyond its detection range. More specifically, the cuboid length L_(cuboid) shall in part be governed by the minimum required POT; the cuboid length L_(cuboid) shall not be so great that the target to be detected is smaller than the minimum required POT.

Referring now particularly to FIG. 2, the cuboid 23 has a width W_(cuboid) extending outwardly, or as it is defined now, “outboard” of the vessel 13 from the exterior 16. On the inner side of the cuboid 23, the cuboid 23 is defined by a vertical plane against the exterior 16 of the starboard side of the vessel 13, or coincident with the exterior 16 of the vessel 13. The cuboid width W_(cuboid) will vary in correspondence with the cuboid length L_(cuboid) as long as it is effective despite a potential maximum expected ship list of nine degrees. The cuboid length L_(cuboid) can be shortened if an increased cuboid width W_(cuboid) is needed to accommodate ship list.

The cuboid length L_(cuboid) shall be an order of magnitude larger than each of the cuboid width W_(cuboid) and cuboid height H_(cuboid) (150 meters, 12 meters, and 8 meters, respectively, for instance).

The various dimensions of the cuboid are determined according to:

$\begin{matrix} {{W_{cuboid} = {h_{c}\cot\;\alpha\;\tan\;\beta}},} & (1) \\ {{H_{cuboid} = h_{c}},{and}} & (2) \\ {{L_{cuboid} = {D - {2h_{c}\cot\;\alpha}}},} & (3) \end{matrix}$

where:

h_(c) is the height of the camera from the waterline 15;

N is the horizontal resolution of the camera 11 or 12;

M is the vertical resolution of the camera 11 or 12;

D is the distance between the cameras 11 and 12;

α is the angular dimension of the HFOV; and

β is the angular dimension of the VFOV.

It is noteworthy that the arrangement of paired opposing cameras 11 and 12 is merely intended to detect overboard falls in the defined cuboid 23 between the cameras 11 and 12, and not outside of or beyond that cuboid 23. The above equations (1), (2), and (3) determine the dimensions of the cuboid 23 assuming a zero ship list angle. Nonetheless, W_(cuboid) as defined above, is applicable for moderate ship lists of preferably between three and five degrees and up to nine degrees on the exemplary vessel 13. Additional coverage can be achieved by the introduction of additional camera pairs and their respective detection cuboids, as FIGS. 4-9 show.

FIG. 3 shows the principal processing stages of the system 10 in the context of a functional block diagram. The broken line input indicates a preference for use of cameras with an external trigger mode when available. Cameras with an external trigger mode will initiate recording contemporaneously, and temporal correlation of corresponding frames from opposing cameras is desired for use in the system 10. Without temporal correlation, frames captured from opposing cameras 11 and 12 are time-stamped upon being digitized in the case of analog cameras, in step 101. It is noted that two of the functional blocks are identified as step 101, because the function specified in step 101 is performed on the images from the aft camera 11 and the aft camera 12. Operating at 30 frames per second (“fps”) entails an uncertainty in time of at most 16.5 milliseconds, which is tolerable within the system 10. After time stamping, the video is communicated to a central server on the vessel 13. To avoid undue degradation of the acquired video during communication across long distances between the cameras 11 and 12 and the server, optical fibers are preferably used to transmit the video data. However, if the video for analysis is to be encoded, it is preferably subjected only to intra-frame encoding (such as with MJPEG encoding), and is preferably lossless. A low-compression ratio version of, for example, MJPEG encoding, is used for archival purposes.

The time-stamped frames of the opposing cameras 11 and 12 are next dumped into their respective first-in, first-out memory (“FIFO”) buffers at step 102. The sequence of processing steps 103-106 after step 102 continually repeats itself, with each subsequent temporally-corresponding image pair from each of the FIFO memory buffers in step 103. This sequence of operation progresses at a 30 fps rate.

Following step 103, the image pair is preprocessed and segmented in step 104. Preprocessing is the operation of classifying pixels into background and foreground, through a multimodal background model that exploits both intensity and gradient orientation. This operation is described in U.S. patent application Ser. No. 14/210,435, entitled Background Modeling For Fixed, Mobile, and Step-and-State Video Camera Surveillance, with filing date of March 2014, and which is hereby incorporated by reference. Morphological filtering, also described in the '435 application, is performed after classification of pixels.

Segmentation is directed to partitioning the image into meaningful regions. In other words, segmentation partitions the image into clustered sets of foreground pixels which show a degree of cohesion corresponding to sought objects including the number of clustered pixels. When needed, as determined from the appearance of a sufficiently large number of clustered pixels (hereinafter referred to as a “blob” for ease of reference) each representing an object in image space, the processing operation that includes background modeling, pixel classification, and morphological filtering that exploit motion is augmented by an intra-frame segmentation operation. The intra-frame segmentation operation extracts moving or stationary objects from a single image, rather than employing a background model whose construction requires a sequences of images.

To prevent the loss or overlooking of any potential target which may be revealed through segmentation, the system 10 next concurrently intertwines an object pairing operation with a tracking operation, as shown in step 105. Concurrently performing these operations is mode demanding on computer processing power, but it does provide increased reliability in determining true MOB events. The use of the detection cuboid 23 allows the process to determine a number of pieces of information about various potential falls. These pieces of information are cues useful in aiding the system 10 in determining whether an MOB incident has occurred or not.

More specifically, for each paired object, the system can determine the following information or cues:

-   -   i) Approximate world space size of a paired object, as opposed         to the apparent image space size of the paired object;     -   ii) Approximate world space location of a paired object with         respect to the pair of opposing cameras 11 and 12, and thus,         also with respect to the vessel 13;     -   iii) Approximate world space speed and acceleration of a paired         object. An authentic fall should, in both images, exhibit         substantial speed and acceleration predominantly in downward or         vertical direction; and     -   iv) Pairing of track segments in order to derive an approximate         trajectory of the fall in world space with respect to the         exterior 16 of the vessel 13.

The above cues are then analyzed in the context of a variety of potential falls from the highest and lowest decks in Table 1 below:

TABLE 1 Source deck Speed* at number Height lowest Deck (source of above (i.e. camera) Speed* at Monitored fall) waterline level waterline time* 11 35 m 21 m/s 26 m/s 0.5 sec (115 ft) (47 mph) (58 mph) (15 frames) 3 12 m 0 15 m/s 1.5 sec (40 ft) (34 mph) (45 frames) *Assumes fall in vacuum, though actual falls are only marginally slower. By comparing the above determined cues with figures such as those presented in Table 1 above, an authentic MOB event can be determined.

Intertwined or concurrent tracking and pairing according to step 105 includes a number of processing steps. The following symbols and variables are used in those processing steps, which are described afterward. Some of these symbols and variables have been defined before but are repeated here to refresh the memory.

h_(Fall) Source deck height above the water line

h_(c) Camera height

h_(monitored) Monitored height

τ_(monitored) Computationally-expected monitored time

fps Frame rate

N Horizontal resolution of camera 11 or 12

M Vertical resolution of camera 11 or 12

D Distance between pair of opposed cameras 11 and 12

α Angular HFOV

β Angular VFOV

n_(Fwd) Number of pixels claimed or covered by the width of an object in an image captured by the forward camera 11

m_(Fwd) Number of pixels claimed or covered by the height of an object in an image captured by the forward camera 11

n_(AFT) Number of pixels claimed or covered by the width of an object in an image captured by the aft camera 12

m_(Aft) Number of pixels claimed or covered by the height of an object in an image captured by the aft camera 12

x_(Fwd) X-coordinate of origin of a bounding box drawn around an object in an image captured by the forward camera 11

y_(Fwd) Y-coordinate of origin of a bounding box drawn around an object in an image captured by the forward camera 11

x_(Aft) X-coordinate of origin of a bounding box drawn around an object in an image captured by the aft camera 12

y_(Aft) Y-coordinate of origin of a bounding box drawn around an object in an image captured by the aft camera 12

H_(cuboid) Height of cuboid 23

W_(cuboid) Width of cuboid 23

L_(cuboid) Length of cuboid 23

r_(fn) Range based on width of time-stamped paired objects captured by the forward camera 11

r_(an) Range based on width of time-stamped paired objects captured by the aft camera 12

r_(fm) Range based on height of time-stamped paired objects captured by the forward camera 11

r_(am) Range based on height of time-stamped paired objects captured by the aft camera 12

r_(f) Range from forward camera 11 when a potential pair exhibits range conformance via width and height cues

r_(a) Range from aft camera 12 when a potential pair exhibits range conformance via width and height cues

α_(f) Angular subtense of the width of the object captured by the forward camera 11

α_(a) Angular subtense of the width of the object captured by the aft camera 12

β_(f) Angular subtense of the height of the object captured by the forward camera 11

β_(a) Angular subtense of the height of the object captured by the aft camera 12

W_(t) World space width of object

H_(t) World space height of object

A_(t) World space area of object

World space xy coordinates and image space ji coordinates in the pair of opposed cameras 11 and 12 are related according to: j _(Fwd) =x _(Fwd) and i _(Fwd) =M−y _(Fwd)−1 j _(Aft) =N−x _(Aft)−1 and i _(Aft) =M−y _(Aft)−1 The origins and orientations of the axes are different in world space xy coordinates and image space ji coordinates. The ji axes are located at the respective camera 11 or 12, with the j-axis aligned with the camera scan line towards the opposing camera 12 or 11, and with the i-axis perpendicular to the scan line. In the xy coordinate system, the axes are attached to a top-left corner of the image and extend along the scan line and perpendicular to it, respectively. The following variables are used:

xo_(Fwd) The x component of the object bounding box centroid in xy image coordinates pertaining to the forward camera 11 in object pairing or track point-pair pairing, respectively

yo_(Fwd) The y component of the object bounding box centroid in xy image coordinates pertaining to the forward camera 11 in object pairing or track point-pair pairing, respectively

jo_(Fwd) The j component of the object bounding box centroid in ji image coordinates pertaining to the forward camera 11 in object pairing or track point-pair pairing, respectively

io_(Fwd) The i component of the object bounding box centroid in ji image coordinates pertaining to the forward camera 11 in object pairing or track point-pair pairing, respectively

xo_(Aft) The x component of the object bounding box centroid in xy image coordinates pertaining to the aft camera 12 in object pairing or track point-pair pairing, respectively

yo_(Aft) The y component of the object bounding box centroid in xy image coordinates pertaining to the aft camera 12 in object pairing or track point-pair pairing, respectively

jo_(Aft) The j component of the object bounding box centroid in ji image coordinates pertaining to the aft camera 12 in object pairing or track point-pair pairing, respectively

io_(Aft) The i component of the object bounding box centroid in ji image coordinates pertaining to the aft camera 12 in object pairing or track point-pair pairing, respectively

(ικλ) A 3-dimensional Cartesian-coordinate system representing real distances from its origin. The origin of this coordinate system lies at the midpoint of the pair of opposing cameras 11 and 12 at the level of the cameras 11 and 12, with distances measured increasingly positive towards the forward camera 11, and with outboard and downward distances also being measured in positive values. The following variables are used:

ι_(Fwd) A component representing real distance from the origin (i.e., from a midpoint between the cameras 11 and 12)

k_(Fwd) A component representing outboard distance from the origin (i.e., from the exterior 16 of the vessel 13)

λ_(Fwd) A component representing downward distance from the origin (i.e., from the level of the cameras 11 and 12)

ι_(Aft) A component representing real distance the origin (i.e., from a midpoint between the cameras 11 and 12)

k_(Aft) A component representing outboard distance from the origin (i.e., from the exterior 16 of the vessel 13)

λ_(Aft) A component representing downward distance from the origin (i.e., from the level of the cameras 11 and 12)

tk_(ptpairTilrFwd) Track point-pair tilt angle in image space of the forward camera 11

tk_(ptpairTilrAft) Track point-pair tilt angle in image space of the aft camera 12

Much like (ικλ), (ι_(tr),κ_(tr),λ_(tr)) represents a 3-dimensional Cartesian coordinate system representing real distances from the same origin. However, instead of being derived from the width and height of the paired object bounding boxes in pixel space, coordinates in this system are derived from projections of two track segments in image space, along the camera scan line and perpendicular to it, for timestamp-paired images in both opposing cameras 11 and 12. The pairing is done between points in the fall trajectory, or “track points,” whose timestamps conform and their underlying blob has survived the initial pruning in the course of segmentation from step 104. The track points are the centroids of the object, or of the bounding box surrounding the objects, in the image space of each of the cameras 11 and 12. Two track points establish a track segment, as they represent a segment of the trajectory of the fall in image space. Thus, for this processing stage, two timestamp-paired images from each camera 11 and 12 are to be tested for conformance. The track point pair need not include immediately preceding and succeeding frames, rather, they may be selected from one frame to variously distant frames in time. Indeed, the track points are paired combinatorially and analyzed to reduce error.

Conforming track segments, when not too close in time, do not exhibit disparate tilt angles

${\tan^{- 1}\left( {\frac{\delta\; y}{\delta\; x}} \right)}.$ Such conformance can optionally be examined through a Hough-like transform of candidate track points, with each track point pair potentially defining a candidate track segment. Outliers are discarded and the extent of conformity of the survivors is verified. For this operation, the following variables are used:

(ι_(tr))_(Fwd) A component representing real distance from the origin (i.e., the midpoint between the cameras 11 and 12) toward the forward camera 11;

(κ_(tr))_(Fwd) A component representing real outboard distance from the origin toward the forward camera 11;

(λ_(tr))_(Fwd) A component representing real downward distance from the origin toward the forward camera 11;

βo_(Fwd) Angular subtense of outboard distance manifested by a blob imaged by the forward camera 11, or equally, that pertains to two track points (i.e., a track segment) at different frames;

βo_(Aft) Angular subtense of outboard distance manifested by a blob as observed by the aft camera 12, or equally, that pertains to two track points (i.e., a track segment) at different frames;

αo_(Fwd) Angular subtense of downward distance manifested by a blob as observed by the forward camera 11, or equally, that pertains to two track points (i.e., a track segment) at different frames; and

αo_(Aft) Angular subtense of downward distance manifested by a blob as observed by the aft camera 12, or equally, that pertains to two track points (i.e, a track segment) at different frames.

Concurrent tracking and pairing as shown in step 105 of FIG. 3 follows image segmentation, which entails an implicit pruning (i.e. discarding of blobs which defy, say, expected size constraints, or a combination of size and location constrains, or when the opposing timestamp consistent frame does not exhibit any blob).

To reduce the vulnerability of processing to erroneous objects that may survive the segmentation process, the methodology offers multiple cues to be exploited to meet such occurrences. Such erroneous objects could include a plane in the distance, a bolt of water jettisoned from the deck, a tugboat pulling alongside the vessel 13, or a passenger boarding the vessel 13 on a gangway far below. First, the spans or dimensions of a bounding box for an object are compared. Second, those spans or dimensions are transformed into world space sizes and compared to estimated real object width and height and, hence, size. Third, pairing of track point pairs (i.e., a track segment) in timestamp-paired frames from the opposing cameras 11 and 12 yields additional cues that can be exploited in discarding erroneous blobs.

When pairing objects through their appearance in the cameras 11 and 12, the width and height of the bounding boxes surrounding the objects are used to arrive at ranges of the objects from the cameras 11 and 12. The width and height of the bounding boxes are measured in pixels along the scan line of the cameras 11 and 12 and perpendicular to the scan line. This information, when combined with the known subtending angles for the width and height, allows the system 10 to determine the world space ranges. For authentic objects, these ranges should conform to an estimate, and should be less than the inter-camera distance D. Nevertheless, the range is merely used as a cue, with classification in step 106 of FIG. 3 being where the ultimate determination of whether an MOB event has occurred or not. In step 106, multiple cues are compiled, such as object width, object height, track point pairing, and others.

The ranges are calculated according to the following algorithms. The range based on the width of a bounding box of an object in timestamp-paired frames from the forward camera 11 is determined by:

$\begin{matrix} {r_{fn} = {\frac{n_{Aft}}{n_{Fwd} + n_{Aft}} \cdot {D.}}} & (4) \end{matrix}$ The range based on the width of a bounding box of an object in timestamp-paired frames from the aft camera 12 is determined by:

$\begin{matrix} {r_{an} = {\frac{n_{Fwd}}{n_{Fwd} + n_{Aft}} \cdot {D.}}} & (5) \end{matrix}$ The range based on the height of a bounding box of an object in timestamp-paired frames from the forward camera 11 is determined by:

$\begin{matrix} {r_{fm} = {\frac{m_{Aft}}{m_{Fwd} + m_{Aft}} \cdot {D.}}} & (6) \end{matrix}$ The range based on the height of a bounding box of an object in timestamp-paired frames from the aft camera 12 is determined by:

$\begin{matrix} {r_{am} = {\frac{m_{Fwd}}{m_{Fwd} + m_{Aft}} \cdot {D.}}} & (7) \end{matrix}$ The above equations (4)-(7) rely on the following relationships:

$\begin{matrix} {{\alpha_{fm} \approx {\frac{n_{Fwd}}{N} \cdot \alpha}},} & (8) \\ {{\alpha_{an} \approx {\frac{n_{Aft}}{N} \cdot \alpha}},} & (9) \\ {{\beta_{fm} \approx {\frac{m_{Fwd}}{M} \cdot \beta}},{and}} & (10) \end{matrix}$

$\begin{matrix} {\beta_{am} \approx {\frac{m_{Aft}}{M} \cdot {\beta.}}} & (11) \end{matrix}$ The cameras 11 and 12 will record approximately the same height. With an approximately similar height, the following equation is written: β_(a) r _(a)≈β_(f) ·r _(f)  (12). Further, the cameras 11 and 12 will also generally capture approximately the same width. With an approximately similar width, the following equation is written: α_(a) r _(a)≈α_(f) ·r _(f)  (13). Because the object must be between the forward camera 11 and the aft camera 12, the following height-derived range and the width-derived range must satisfy the following constraint: r _(a) +r _(f) ≈D  (14). Further, conformance is determined when the height-derived range and the width-derived range deviate from each other by no more than a user-defined tolerable extent.

Substituting for β_(f) and β_(f) in (12) from (10) and (11) yields a relationship for the height:

$\begin{matrix} {{{\frac{m_{Aft}}{M} \cdot \beta \cdot r_{a}} \approx {\frac{m_{Fwd}}{M} \cdot \beta \cdot r_{f}}},} & (15) \end{matrix}$ or, more simply: m _(Aft) ·r _(a) ≈m _(Fwd) ·r _(f)  (16). Equation (16) states that the number of pixels claimed or covered by the height of a bounding box for a falling object in a corresponding image pair from the cameras 11 and 12 is approximately inversely proportional to their ranges from the respective camera.

Similarly, for width: n _(Aft) ·r _(a) ≈n _(Fwd) ·r _(f)  (17).

Finally, by substituting for r_(f) in (14) and rearranging, equations for forward camera 11-based range and aft camera 12-based range are obtained from object width as follows:

$\begin{matrix} {{r_{fn} \approx {\frac{n_{Aft}}{n_{Aft} + n_{Fwd}} \cdot D}},{and}} & (18) \\ {r_{an} \approx {\frac{n_{Fwd}}{n_{Aft} + n_{Fwd}} \cdot {D.}}} & (19) \end{matrix}$

Similarly, equations for forward camera 11-based range and aft camera 12-based range are obtained from object height as follows:

$\begin{matrix} {{r_{fm} \approx {\frac{m_{Aft}}{m_{Aft} + m_{Fwd}} \cdot D}},{and}} & (20) \\ {r_{am} \approx {\frac{m_{Fwd}}{m_{Aft} + m_{Fwd}} \cdot {D.}}} & (21) \end{matrix}$

A conforming paired object as observed from the forward and aft cameras 11 and 12 must yield approximately the same ranges from corresponding cameras 11 and 12 based on both object width and height. If the object does not demonstrate conformance, the object is either dismissed as not being due to the same actual object, or is negatively scored. On the other hand, if the ranges conform, then aggregate ranges from the forward and aft cameras 11 and 12 for that object are computed with equations (20) and (21). These computed ranges, in turn, are used to estimate world space width and height of the object, which is then compared to the expected width and height of a falling person. It should be remembered that a falling person may exhibit different widths and heights due to their presented aspects to the camera pair, and due to movement and rotation of the person during the fall.

The rigidity of dismissal criteria may in some cases be relaxed when considering several contributing point pairs across different frames. Such dismissal criteria may be relegated to a scoring mechanism which considers potential different track point pairs across the time of descent.

In step 107, pertinent information is provided to the bridge of the vessel 13 to assist in a recovery operation. The system determines the location of the object. The location in the 3-dimensional Cartesian coordinate system (ικλ) is determined.

The component along the ι-axis (i.e., along the camera line) is given by:

$\begin{matrix} {{l \approx \frac{l_{Fwd} + l_{Aft}}{2}},{Where}} & (22) \\ {{l_{Fwd} \approx {\frac{D}{2} - r_{f}}},{and}} & (23) \\ {l_{Aft} \approx {r_{f} - {\frac{D}{2}.}}} & (24) \end{matrix}$

The component along the k-axis (i.e., the real outward distance from the exterior 16 of the vessel 13) is given by:

$\begin{matrix} {{\kappa \approx \frac{k_{Fwd} + k_{Aft}}{2}},} & (25) \end{matrix}$ where κ_(Fwd) ≈r _(f) ×·βo _(Fwd)  (26), κ_(Aft) ≈r _(a) ×·βo _(Aft)  (27), and where

$\begin{matrix} {{{\beta\; o_{Fwd}} \approx {\frac{{io}_{Fwd}}{M} \times \beta}},} & (28) \end{matrix}$ and

$\begin{matrix} {{\beta\; o_{Aft}} \approx {\frac{{io}_{Aft}}{M} \times {\beta.}}} & (29) \end{matrix}$

The downward component of the fall along the λ-axis is given by:

$\begin{matrix} {{\lambda \approx \frac{\lambda_{Fwd} + \lambda_{Aft}}{2}},} & (30) \end{matrix}$ where λ_(Fwd) ≈r _(f) ×·αo _(Fwd)  (31), λ_(Aft) ≈r _(a) ×·αo _(Aft)  (32), and where

$\begin{matrix} {{{\alpha\; o_{Fwd}} \approx {\frac{N - {jo}_{Fwd}}{N} \times \alpha}},} & (33) \end{matrix}$ and

$\begin{matrix} {{\alpha o}_{Aft} \approx {\frac{N - {jo}_{Aft}}{N} \times {\alpha.}}} & (34) \end{matrix}$

The above equations (22)-(34) show that for every conformant object pair, a world space location with respect to the pair of opposed cameras 11 and 12 and, thus, also with respect to the vessel 13, can be determined. Moreover, from a collection of such points, the system 10 determines a trajectory in world space for the fall and assesses the trajectory and the segments thereof to see if they meet an expected trajectory, speed, and acceleration profile of an authentic MOB fall. If they do approximate an expected trajectory, speed, and acceleration profile of an authentic MOB fall, then the system 10 identifies the event as an MOB. The cues then support the determination to increase its reliability.

Much like the manner in which ranges from the paired object width and height are compared, ranges from pairing track segments in opposing cameras 11 and 12 may also be derived and compared for conformance. Comparison is made both along the scan line and perpendicular to the scan line.

First, world space coordinates are derived for conforming track segments:

$\begin{matrix} {{\left( l_{tr} \right)_{Fwd} \approx \frac{l_{Fwd} + l_{Aft}}{2}},} & (35) \end{matrix}$ where

$\begin{matrix} {{\left( l_{tr} \right)_{Fwd} \approx {\frac{D}{2} - r_{f}}},} & (36) \end{matrix}$ and

$\begin{matrix} {\left( l_{tr} \right)_{Aft} \approx {r_{f} - {\frac{D}{2}.}}} & (37) \end{matrix}$

The component along the k-axis (i.e., the world space outboard distance from the exterior 16 of the vessel 13) is given by:

$\begin{matrix} {{\left( \kappa_{tr} \right)_{Fwd} \approx \frac{k_{Fwd} + k_{Aft}}{2}},} & (38) \end{matrix}$ where (κ_(tr))_(Fwd) ≈r _(f) ×·βo _(Fwd)  (39), and (κ_(tr))_(Aft) ≈r _(a) ×·βo _(Aft)  (40), and where

$\begin{matrix} {{{\beta\; o_{Fwd}} = {\frac{{io}_{Fwd}}{M} \times \beta}},} & (41) \end{matrix}$ and

$\begin{matrix} {{\beta\; o_{Aft}} = {\frac{{io}_{Aft}}{M} \times {\beta.}}} & (42) \end{matrix}$

The downward component of the fall along the λ-axis is given by:

$\begin{matrix} {{\left( \lambda_{tr} \right)_{Fwd} \approx \frac{\lambda_{Fwd} + \lambda_{Aft}}{2}},} & (43) \end{matrix}$ where (λ_(tr))_(Fwd) ≈r _(f) ×·αo _(Fwd)  (44), and (λ_(tr))_(Aft) ≈r _(a) ×·αo _(Aft)  (45), and where

$\begin{matrix} {{{\alpha\; o_{Fwd}} = {\frac{N - {j\; o_{Fwd}}}{N} \times \alpha}},} & (46) \end{matrix}$ and

$\begin{matrix} {{\alpha\; o_{Aft}} = {\frac{N - {j\; o_{Aft}}}{N} \times {\alpha.}}} & (47) \end{matrix}$

The equations above provide the ability to derive conformant track segments and qualify them with conformant width and height (and thus, also area) information about the corresponding object. This information, in isolation or combination, is used to provide the necessary attributes to determine whether an MOB event has occurred or not.

While the above disclosure has been made with respect to thermal video cameras, one having ordinary skill in the art will appreciate that under some conditions, visible-light video cameras. In some situations, visible-light video cameras may be sufficient when equipped with infrared illuminators of different wavelengths for dark hours. Appropriate short-pass and long-pass infrared filters would be used to avoid interference with the operation of the opposing cameras. Further, in some cases, an arrangement involving LIDAR sensors, rather than video cameras, may be used as long as the issues raised by their interference is known and accounted for.

This description has heretofore referred only to the cameras 11 and 12 as exemplary cameras for the system 10. However, the system 10 is not intended to be used with only two cameras. Rather, a preferred embodiment of the system 10 includes cameras mounted around the entire perimeter of the vessel 13 to provide monitoring around the entire perimeter of the vessel 13, rather than a section thereof. As mentioned earlier, additional coverage is achieved by the introduction of additional camera pairs and their respective detection cuboids, as FIGS. 4-9 show. FIG. 4 illustrates the exterior 16 of the starboard side of the vessel 13, which is mounted with a plurality of cameras 30-37. The cameras 30-37 include wide- and narrow-field camera which are chosen, mounted, or oriented for different reasons. The cameras 30-37 form different pairs of cameras as well. However, it is noted that for purposes of this embodiment, pairs are formed only between cameras having identical fields of view and resolutions; the processes described above rely on opposing pairs of identical cameras. Nevertheless, there are many pairs. For instance, because camera 30 is directed toward the bow 19 of the vessel 20, and the camera 31 is directed toward the stern of the vessel 13, and those cameras 30 and 31 have identical characteristics (both cameras 30 and 31 are wide-field, high resolution cameras), they form a pair. Camera 36, which is located proximate to camera 30, has a narrow field of view and is directed forward. Camera 33 and camera 37 (which is located proximate to camera 35) are each directed rearward and each form a pair with camera 36. Thus, the view volumes of the cameras 30-37 overlap and create multiple detection cuboids for different camera pairs. The cameras 36 and 37 are mounted proximate to the cameras 30 and 35, respectively, but instead of having an extremely wide field of view, have a narrow field of view, such as approximately 18°, which provides a different field of view at that location.

Some of the cuboids formed by the various pairs of the cameras 30-37 are contiguous, meaning they are adjacent and located in juxtaposition end-to-end, while other of the cuboids formed by the cameras 30-37 are overlapping, meaning that they extend into or over each other by some portion. Some of the detection cuboids are shown in FIG. 5.

Referring to FIG. 5, a cuboid 40 is formed between the opposed cameras 30 and 31. As can be seen by their view volumes, the cameras 30 and 31 each have an extremely wide field of view, such as 110°. This allows the cuboid 40 to have a very high occupancy, meaning that the cuboid 40 occupies a very large portion of the overlap of the view volumes of the cameras 30 and 31; here, in fact, the cuboid 40 occupies the entire overlap. This is useful proximate to the stern 20 of the vessel 13 where it is difficult to mount a camera. Without extremely wide field-of-view cameras 30 and 31 near the stern 20, the cuboid 40 would be much shorter, and would expose a detection gap, through which a passenger could fall without triggering an MOB alarm.

Closer to the bow 19, a cuboid 43 is formed between the opposed cameras 34 and 35. Like the cameras 30 and 31, the cameras 34 and 35 also have an extremely wide field of view.

Between the cuboids 40 and 43, three cuboids 41, 42, and 44 are formed. The cuboids 41 and 42 are contiguous to each other, and together are contiguous with the cuboids 40 and 43. The cuboid 41 is formed between the forwardly-directed narrow-field camera 36 and the rearwardly-directed camera 33. Similarly, the cuboid 42 is formed between the rearwardly-directed narrow-field camera 37 and the forwardly-directed camera 32. The cuboids 44 and 45 each overlap with the cuboid 44.

The cuboid 44 is formed between the pair of cameras 36 and 37. These cameras 36 and 37 have narrow fields of view but low resolution capabilities compared to the wide field-of-view cameras 30 and 35, allowing for greater ranges with comparable POT as those of the cameras 30 and 35. The cuboid 44 illustrates an overlapping detection volume. The cuboid 44 is shown elevated above the cuboids 41 and 42 for purposes of clarity of the illustration only; in fact, the entirety of the cuboid 42 overlaps portions of the cuboids 41 and 42. The cuboid 44 thus provides redundant monitoring of portions of the cuboids 41 and 42.

Referring now to FIG. 6, the cuboids 40-44 are shown in a top plan view illustrating the detection volumes as they extend outboard from the exterior 16 of the vessel 13. The cuboids 44 is again shown offset for clarity of the illustration.

Turning to FIG. 7, the stern 20 of the vessel 13 is shown in top plan view. Two cameras 50 and 51 are mounted just off the stern 20 of the vessel and directed somewhat toward each other. The view volumes of the cameras 50 and 51 are necessarily limited in that they cannot “see around” the stern 20 and are thus limited in coverage by the obstruction of the stern 20 itself. Hence, the leading edges of the view volumes of the cameras 50 and 51 extend across the exterior 16 of the stern 20. This leaves a gap 52 between the view volumes and the stern 20. An alternate camera configuration is shown in FIG. 8 formed by four cameras 60, 61, 62, and 63. The cameras 60 and 63 are mounted at the starboard and port side of the stern 20, respectively, and the cameras 61 and 62 are mounted at the same location therebetween; the cameras 61 and 62 are directed generally away from each other. Even this arrangement creates a gap 64 in the view volumes, albeit smaller than the gap 52 shown in FIG. 7.

Referring now finally to FIG. 9, a top plan view of the bow 19 illustrates coverage from two cameras 70 and 71. The opposed cameras 70 and 71 are wide-angle cameras mounted apart from each and oriented in a converging fashion. In this embodiment, the cameras 70 and 71 have fields of view of approximately 90°. As is seen in this FIG. 9 and FIGS. 7-8, exterior convex surfaces are demanding in terms of the number and type of cameras needed to cover and monitor the exterior 16.

A preferred embodiment is fully and clearly described above so as to enable one having skill in the art to understand, make, and use the same. Those skilled in the art will recognize that modifications may be made to the described embodiment without departing from the spirit of the invention. To the extent that such modifications do not depart from the spirit of the invention, they are intended to be included within the scope thereof. 

The invention claimed is:
 1. A method of detecting a man overboard event on a structure above a waterline, the method comprising the steps of: providing first and second imaging devices, each having a view volume, contemporaneously processing first and second video streams of a common detection volume, the common detection volume characterized as a three-dimensional space disposed above the waterline, along the structure, and formed from an overlap of the view volumes of the first and second imaging devices; contemporaneously monitoring with the first and second imaging devices a falling object falling along a trajectory through the common detection volume in the first and second video streams; determining, from the first and second video streams, corresponding first and second characteristics, respectively, of the falling object as the object falls along the trajectory through the common detection volume streams, respectively; and comparing the first and second characteristics from the first and second video streams and identifying a man overboard event in response to a conforming comparison.
 2. The method of claim 1, wherein the step of comparing includes comparing the first characteristic from the first video stream to the second characteristic from the second video stream.
 3. The method of claim 1, wherein the step of comparing includes comparing the first and second characteristics from the first and second video streams with an expected characteristic.
 4. The method of claim 1, wherein the first and second characteristics include at least one of a height of the falling object, a width of the falling object, a first range of the falling object from the first camera, a second range of the falling object from the second camera, a velocity of the falling object, an acceleration of the falling object, the trajectory of the falling object, a segment of the trajectory of the falling object, and a position of the falling object.
 5. The method of claim 1, wherein the common detection volume is limited vertically to extending between the waterline and a lowest height from which a fall could originate from the structure.
 6. The method of claim 1, wherein the first and second imaging devices are thermal cameras.
 7. The method of claim 1, wherein the first and second imaging devices have larger fields of view in a vertical direction than in a horizontal direction.
 8. The method of claim 1, wherein the first and second imaging devices are oriented so that the view volumes of the first and second imaging devices extend across an exterior surface of the structure.
 9. The method of claim 1, further comprising the step of transmitting an alarm, the alarm including portions of the first and second video streams including the man overboard event, a time of the man overboard event, and a location of the man overboard event.
 10. A method of detecting a man overboard event on a structure above a waterline, having an exterior, and having a plurality of decks including a lowest deck, the method comprising the steps of: providing first and second imaging devices contemporaneously processing first and second video streams, respectively, of a common detection volume characterized as a three-dimensional space disposed above the waterline, wherein the first imaging device has a first view volume, and the second imaging device has a second view volume which overlaps the first view volume to define the common detection volume; contemporaneously monitoring in the first and second video streams an object falling along a trajectory through the common detection volume; processing the first and second video streams to determine from the first and second video streams determined characteristics of the falling object as the object falls along the trajectory through the common detection volume; and identifying a man overboard event in response to the determined characteristics of the object in the common detection volume conforming to expected characteristics as the object falls along the trajectory through the common detection volume.
 11. The method of claim 10, wherein the determined characteristics include at least one of a height of the object, a width of the object, a first range of the object from the first camera, a second range of the object from the second camera, a velocity of the object, an acceleration of the object, the trajectory of the object, a segment of the trajectory of the object, and a position of the object.
 12. The method of claim 10, wherein the first and second imaging devices have larger fields of view in a vertical direction than in a horizontal direction.
 13. The method of claim 10, wherein the common detection volume is bound between the waterline and the lowest deck.
 14. The method of claim 10, wherein the first and second imaging devices are oriented so that the first and second view volumes extend across the exterior of the structure.
 15. A system of detecting a man overboard event on a structure above a waterline, having an exterior, and having a plurality of decks including a lowest deck, the system comprising: first and second imaging devices having first and second view volumes and configured to contemporaneously record a common detection volume in the first and second video streams, respectively; the common detection volume defined in an overlap of the first and second view volumes, characterized as a three-dimensional space bound between the waterline and the lowest deck; and a computer coupled in data communication with the first and second imaging devices, the computer configured to contemporaneously monitor with the first and second imaging device a falling object as the falling object falls along a trajectory through the common detection volume, process the first and second video streams of the first and second imaging devices, respectively, and identify a man overboard event in response to the falling object falling along the trajectory through the common detection volume and exhibiting determined and corresponding characteristics which conform to expected characteristics and to each other.
 16. The system of claim 15, wherein the first and second imaging devices are thermal cameras.
 17. The system of claim 15, wherein the first and second imaging devices have larger fields of view in a vertical direction than in a horizontal direction.
 18. The system of claim 15, wherein the first and second imaging devices are oriented so that the first and second view volumes extend across the exterior of the structure.
 19. The system of claim 15, wherein: the first and second imaging devices are spaced apart from each other by an inter-camera distance; the inter-camera distance is limited by a maximum range of the imaging devices; and the maximum range is limited by a pixel-on-target minimum for the falling object in the common detection volume. 